Changes Involving Genomes and Chromosomes 



141 



drugs like colchicine (which destroy the 

 spindle, so that the anaphase movement of 

 chromosomes is prevented) and environ- 

 mental stresses like starvation and cold can 

 artificially induce autopolyploidy. We have 

 already mentioned the normal occurrence of 

 parthenogenesis in Solenobia and the sub- 

 sequent fusion of haploid and of diploid 

 nuclei to establish diploidy and tetraploidy, 

 respectively. In other organisms, partheno- 

 genesis may be artificially induced and haploid 

 development initiated. In such cases, de- 

 velopment as a haploid, of an individual that 

 is ordinarily diploid, is usually abnormal. 

 The abnormality produced must be due some- 

 times to the expression of detrimental genes 

 which would not be expressed in a diploid 

 because of the presence of their normal alleles 

 on homologous chromosomes. That this is 

 not always the case is evidenced by the fact 

 that if chromosome doubling, naturally or 

 artificially induced, occurs at an early stage, 

 a normal diploid (and homozygous) embryo 

 may be produced. Such a chromosome 

 doubling has produced parthenogenetic sala- 

 manders and rabbits (which are female). In 

 these cases, at least, abnormal development 

 when haploid must often have its basis in 

 quite a different factor. This factor probably 

 involves the surface-volume relationships 

 within the nucleus and between the nucleus 

 and the cytosome, which are disturbed when 

 cells adapted to contain a diploid number of 

 chromosomes carry only a haploid set. 



Autopolyploidy occurs normally in certain 

 somatic cells, such as liver cells in man, and 

 may be induced by the irradiation of cells in 

 tissue culture. In the autopolyploid cells we 

 have so far discussed, each chromosome lies 

 separately in the nucleus and proceeds to the 

 mitotic metaphase independently. There are 

 other cases of autopolyploidy in which all 

 homologous chromosomes are synapsed even 

 though the cell is part of somatic tissue. Let 

 us examine an example of this as found in the 

 giant salivary gland cells of Drosophila larvae. 



Note, first, that in the usual cell of Drosoph- 

 ila, the metaphase chromosome is sausage- 

 shaped, containing chromatids coiled tightly 

 in a series of spirals like those in a lamp 

 filament, and that during interphase the 

 chromatids have largely unwound. The 

 chromosomes in the salivary gland cell 

 nucleus are also in an unwound state, perhaps 

 even more so than in ordinary interphase, 

 and have undergone three special changes. 

 First, each chromosome present has rep- 

 licated synchronously a number of times in 

 succession, so that one chromosome produced 

 two, two produced four, four produced eight, 

 eight formed 16, 16 formed 32, etc. This 

 replication can occur as many as nine times, 

 producing 512 chromosomes. Second, all 

 sister strands, instead of separating, remain 

 in contact with the homologous loci apposed, 

 giving the appearance of a many-threaded, 

 polytene, cable. Third, since the original 

 members of a pair of homologous chromo- 

 somes are paired at homologous points, by 

 what is called somatic synapsis, a double 

 cable is formed which can contain as many 

 as 1024 chromosomes. When seen under the 

 microscope (Figure 18-3), these double 

 cables have a cross-banded appearance. Be- 

 cause of differences in density along the 

 length of the unwound chromosome, a band 

 is formed by a dense part of one chromosome 

 being synapsed to the homologous region of 

 all the other strands of that type (Figure 18-4). 

 The pattern of bands is sufficiently constant 

 and so characteristic that it is possible to 

 identify not only each chromosome but dif- 

 ferent regions within a chromosome (Figure 

 18-5). The giant size of salivary chromo- 

 somes, long because they are unwound, and 

 thick because of synapsed polytenes, offers a 

 unique opportunity to correlate genetical 

 and cytological events. 



While we have so far discussed only auto- 

 polyploidy, there is a different way by which 

 ploidy may increase. It is sometimes found 

 that the genomes in a given species have been 



